Prior art burglar alarm systems in elemental forms involving glass panels have a disadvantage in that sensing elements on or embedded in the glass are visible, which defeats a fundamental purpose for using glass. For example, thin electrically conductive tapes or strips have been applied to the glass periphery in a substantially complete loop. When the glass is broken, the tape is severed and an electrical circuit is broken. This initiates a chain of electrical events resulting in the production of a signal indicating that the glass has been broken. However, such tapes are usually visible. Therefore, they are positioned only around the periphery of the glass area so as not to obstruct the view. Large areas of glass are unprotected and a skilled burglar, intent on entry, may cut the glass while avoiding the visible electrically conductive tape. Alternatively, a hole may be cut through the glass in an area where the tape is absent, through which the burglar reaches in and short circuits the tape between its entry and exit points on the glass panel. As a result, no alarm is ever produced when the glass is broken even though portions of the tape may be severed. The metallic tape may also be unattractive, as well as obstructive, in the environment of the glass panel.
Recently, systems have been devised which respond to sounds or other vibrations produced by intruders. Unfortunately, normal ambient sounds and vibrations may be at a higher level than those caused by a burglar breaking a glass panel. Thus, many false alarms may result in such systems.
What is needed is a security window which provides protection over the entire window area and which is invisible or substantially invisible to the human eye.
Accordingly, it is an object of this invention to provide an improved security window which contains integral intruder detection sensors which are not visible.
Another object of the invention is to provide an improved security window which contains intrusion detection sensors over any desired window area without significantly affecting window transparency.
A further object of this invention is to provide an improved security window which is damage resistant and not subject to false alarms in operation.
Yet another object of this invention is to provide an improved security window which uses conventional plastic window materials and is economic to produce.
Generally speaking, in accordance with the present invention, a polymeric security window is provided which is especially suitable for applications requiring excellent transparency. The security window is a lamination of a pair of polymeric window panels having sandwiched therebetween electrically conductive rods for transmission of electric current therethrough. The primary current transmission medium is composed of coated glass fibers extending from one edge of the plastic window panels to the opposite edge and distributed in strips or rods which substantially cover the entire window area. The glass fibers, coated with an electrically conductive material, are sealed in a plastic material which is preferably the same as the polymeric window panels. The index of refraction of the coated glass fibers is selected to be as close as possible to that of the surrounding plastics, so that minimum distortion of images results when viewing objects through the window. For all practical purposes, the electrically conductive rods are therefore invisible.
The electrically conductive rods which form a grid over the transparent window panel are electrically connected in series circuit, and a current source provides a constant current through the circuit. The resistance of the circuit increases when a rod is severed, partially severed, or punctured and at least some of the electrically conductive glass fibers are severed. A change in the magnitude of voltage required to maintain a constant current in the circuit indicates that a window panel and at least some of the glass fibers have been damaged. Alternatively, a constant voltage may be applied to the circuit. In this case, changes in current flow indicate damage to the window panel. Also, where many window panels in different circuits are under surveillance, the status of the windows may be checked sequentially on a time-shared basis. Comparison of present readings against prior readings for the same window panel may also be the basis for indicating when a panel has been damaged and an alarm signal is to be generated.
The electrically conductive rods may be incorporated into polymeric windows now in general use.
Other objects, features and advantages of the invention will be apparent from the specification. This invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will ba exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.